1. Field of the Invention
The present invention relates to a motor control for a motor used as a prime mover of a vehicle.
2. Description of the Prior Art
Motor control of the above-mentioned kind may be divided into one type of control wherein motor torque is used as a variable to be adjusted, namely a torque control, and another wherein a motor revolution at speed is used as a variable to be adjusted, namely a speed control. The torque control is in most cases employed to meet demands of a driver.
If, for example, motor control is to be combined with a so-called Automatic Speed Control Device (ASCD) or it is to be so modified as to depend on a vehicle speed of another vehicle moving ahead, control based on motor speed is needed. Such speed control is also needed in a case where left and right wheels are driven by different motors so as to adjust a speed difference between the left and right wheels thereby to give a so-called Limited Slip Differential (LSD) effect. Thus, there is a demand for a motor systems which can shift between torque control and speed control.
FIG. 9 is a schematic illustrating a conventional motor control arrangement for a motor 107 used as a prime mover of a vehicle. In this system, torque control is employed wherein a torque demand signal T* generated in response to operation of an accelerator 101 is used as a variable to be adjusted. In this case, the torque demand signal T* is supplied to a controller 105. The controller 105 effects an open loop control wherein a motor torque generated by the motor 107 approaches a target value as represented by the torque demand signal T*. If control based on motor speed is required, a switch 103 is operated to take a position wherein instead of the torque demand signal T* a deviation between a desired motor speed signal V* and an actual motor speed, as detected, of the motor 107 is supplied to the controller 105. Then, the controller 105 effects a closed loop control, i.e., feedback control, wherein the motor 107 is controlled in such a direction as to reduce the deviation toward zero.
This motor control has an insufficiency in that there occurs a shock upon making a shift to the torque control mode from the speed control mode, resulting in a--a poor ride feel of the vehicle. This is because closed loop control is interrupted and replaced by the open loop control at the time of shifting and a rapid change in control gain occurs.
Another approach to effect a smooth shift between torque control and speed control is proposed in Japanese Utility Model Application First Publication 63-149105. According to the motor control system disclosed in this Publication, instead of a closed loop control, an open loop control is used to effect motor speed control wherein a running resistance is anticipated in response to a change in motor speed and outputted in terms of torque in such a manner as to keep the motor at about a desired value. Specifically, an accelerator position, a brake position and a motor speed are used for calculation of an electric current amplitude and a torque angle. These values are used as target values. There are calculated instruction values which gradually approack the above target values, respectively. Torque control of the motor is effected based on these instruction values. This known approach is successful in making a smooth shift between torque control and speed control.
However, this known motor control cannot keep the speed at a desired speed value when the vehicle encounters a change in road conditions, such as when moving uphill. Thus, the functions performed by ASCD and LSD cannot be realized in the known motor control.
Accordingly, an object of the present invention is to provide motor control which effects smooth shift between torque control and speed control of a motor and which is capable of realizing the functions of ASCD and LSD.